1. Field of the Invention
The invention relates to a method for the production of pistons having depression edge armoring, for internal combustion engines, in which a heat resistant armoring ring is connected with the piston blank in the region of the depression edge. The armoring ring according to the invention is more heat-resistant armoring compared to a forged piston blank having a combustion depression.
2. The Prior Art
In order to increase the performance of modern internal combustion engines, particularly diesel engines, the compression pressures and thereby the temperatures in the combustion space are constantly being increased. The result of this measure is that after running the engine, oxidation is found on the steel piston having a combustion depression, or on steel piston heads. This oxidation particularly occurs at the edge of the depression, as a function of the operating temperature that was reached. This oxidation can lead to the formation of cracks and thereby to failure of the component. Likewise, material wear at the piston head, along the fuel injection tracks, is also critical, and makes protection against erosion wear necessary. Known solutions for improving this situation are, for example, coating the finished piston with an oxidation-resistant layer along the edge of the depression by plasma-spraying or application welding of more oxidation-resistant materials onto the pre-finished piston.
The production of an aluminum piston is described in Japanese Patent No. JP-A 63256287, in which a trapezoid-shaped ring consisting of a more heat-resistant aluminum alloy is inserted into a combustion depression that is shaped like a truncated cone, in order to armor the depression edge. The ring is connected with the piston by pressure/friction welding. The geometric design of the ring and the combustion depression, however, have the result that material heated during the friction-welding process can escape at the parting only in the direction of the piston head, and the typical welding flash is formed there, since the material that flows in the direction of the bottom of the combustion depression cannot exit, because of the finite expanse of the face of the ring. The result of this is an insufficient joint, which is characterized by air inclusions, i.e. the formation of bubbles.
A solution that circumvents the aforementioned disadvantages is described in German Publication No. DD 260 844 A3, which describes a method for a positive-lock connection of a conically shaped armoring ring and an iron piston. The heat-resistant armoring ring has a bead-shaped ring flange and is connected with the iron piston by friction welding, in such a manner that the heated piston material, which is capable of flow, surrounds the bead-shaped contour of the ring flange and produces a connection between the piston and the armoring ring. However, the ring/piston connection is produced essentially only over the bead surface of the ring flange. Furthermore, the bead-shaped ring flange would be destroyed during removal of the welding flash, for example by means of machining, and thereby the strength of the ring/piston connection is no longer guaranteed.